1. Field of the Invention
The present invention relates to a channel switching system for mobile communication, and more particularly to a channel switching system in the case where a simplified portable telephone terminal moves at high speed.
2. Description of the Related Art
In a mobile communication system in which a radio base station is arranged in each of a plurality of radio zones, a channel switching system for maintaining communication by switching a channel to a radio base station in a remote site is generally effected according to the following procedures when a mobile station moves between the radio base stations.
In the first place, it is detected that the mobile station has moved from, an area of the radio base station with which communication is currently carried out to an area of another radio base station. The radio base station to which the mobile station has moved is then determined to reserve an idle channel in the determined radio base station. A communication channel of the radio base station is finally switched to that of the radio base station to which the mobile station has moved so that communication is directed with a new channel being allocated to the mobile station.
The trigger of channel switching is judged based on intensity of the electric field of the channels.
As an example to which the above-mentioned channel switching technique is applied, there are techniques disclosed in Japanese patent laid-open publication No. 4-70095 and 4-290098.
FIG. 9 is a block diagram for explaining the technique disclosed in Japanese patent laid-open publication No. 4-70095. As shown in the drawing, a service area for mobile communication is constituted by a plurality of radio zones, wherein radio base stations 1a to 1e are provided in the respective radio zones and these radio base stations are connected with a control base station 4. A mobile station 2 communicates with one of these radio base stations and a currently-used channel is switched to another channel of a radio base station in a remote site as the mobile station 2 moves during communication. The channel switching control is performed in the following manner.
The radio base station currently communicating with the mobile station forecasts a radio base station to which the mobile station may subsequently move and informs the mobile station of information about that radio base station. The mobile station which has received the information monitors a receiving level of a control channel of the predicted radio base station to which the mobile station may subsequently move. When the receiving level of the channel from the currently-used radio base station is lowered whereas the receiving level of the channel of the predicted next radio base station which has been monitored increases above a predetermined value, the mobile station requests the radio base station to carry out channel switching and continues communication with the new radio base station.
Further, FIG. 10 is a black diagram for explaining the technique disclosed in Japanese patent laid-open publication No. 4-290098. This technique is such that a plurality of base stations 1 to N are connected with a central station 15 and channel switching is appropriately effected to maintain communication when a mobile device moves between cells formed by the respective base stations. The central station 15 previously stores the receiving electric field intensities at boundaries between the respective cells. A plurality of the base stations measure an electric field intensity of the mobile device to calculate a position of the device, and hence a time taken by the mobile device to reach a boundary between the cells can be calculated to judge a degree of urgency for performing channel switching. The channels are then sequentially switched by determining their priority according to the degree of urgency.
As described above, although various techniques in regard of channel switching system in the mobile communication system have been disclosed, all of them premise to switch to a channel of the radio base station having the strongest electric field intensity.
A transmitting output of a simplified portable telephone typified by a Personal Handy phone System(PHS) is extremely smaller than that of a general portable telephone, and the service area of the radio base station for the simplified portable telephone is so narrow as 150 to 300 meter. In case of adopting a system for changing over to a radio base station having a strongest electric wave at the time of channel switching, if the mobile station moves at high speed, the mobile station has already passed that radio base station when channel switching is performed and channel switching is thereby failed, which may result in disconnection of a phone call. In such a case, even if channel switching is succeeded, since the mobile station passes multiple radio base stations in a short time and channel switching is therefore performed every time the mobile station passes each radio base station, instantaneous disconnection of a phone call due to channel switching often occurs, thereby disabling a satisfactory phone call.